1. Field of the Invention
The present invention relates to an oblique illumination inspection system and method for identifying defects on or within a glass sheet.
2. Description of Related Art
Manufacturers of glass sheets are always trying to design a new and improved inspection system that can be used to identify defects (e.g., scratches, stains, particle inclusions) that are on or within a glass sheet (e.g., liquid crystal display (LCD) glass substrate). One type of inspection system that is currently being used today relies on a bright field illumination technique to detect, characterize, and classify defects on or within the glass sheet. This type of inspection system is described below with respect to FIG. 1 (PRIOR ART).
Referring to FIG. 1 (PRIOR ART), there is shown a traditional bright field illumination inspection system 100 that is currently used to inspect a glass sheet 102 and identify defects 104 (one shown) on or within the glass sheet 102. The inspection system 100 includes a light source 106 (e.g., fiber line light source 106) and a CCD camera 108. The light source 106 and CCD camera 108 are located on opposite sides of the glass sheet 102, and the light source 106 is located on an optic axis 110 of the CCD camera 108. In operation, the light source 106 emits a light beam 112 which passes through a portion of the glass sheet 102. As shown, the CCD camera 108 and in particular a camera lens 109 receives direct light 112 that passed un-deviated through the transparent defect 104. In addition, the CCD camera 108/camera lens 109 receives light 112a (D+) and 112a (D−) that was diffracted by the transparent defect 104. The camera lens 109 focuses the light 112, 112a (D+) and 112a (D−) onto an image plane 111 within the CCD camera 108. The CCD camera 108 then creates an image which is used to detect, characterize and classify the defect 104.
Although this type of lighting affords a compact design and employs industry standard lighting techniques, the image contrast of certain transparent glass defects, such as silica, scratches, and stains, is relatively poor (see FIG. 5). The poor image contrast hinders the characterization and classification of the transparent defect 104, which adversely impacts the quality of the inspection process. This poor image contrast is primarily due to the interference between the light 112 that passed un-deviated through the transparent defect 104 and the light 112a (D+) and 112a (D−) that was diffracted by the transparent defect 104. In this configuration, light 112a (D+) and 112a (D−) respectively have a positive diffraction order D+ and a negative diffraction order D− both of which interfere with the un-deviated light 112 to essentially wash out the contrast of the transparent defect 104 in the image. The two problematic diffraction orders D+ and D− are present because of the symmetry in the bright field lighting technique (i.e. the light source 106 is located on the optic axis 110 of the CCD camera 108). The resulting poor image contrast and other shortcomings associated with the bright field illumination inspection system 100 are addressed by the present invention.